Fix #20856: Serialize Waiting and Evaluating as if null.

This strategy ensures the "serializability" condition of parallel
programs--not to be confused with the data being `java.io.Serializable`.
Indeed, if thread A is evaluating the lazy val while thread B attempts
to serialize its owner object, there is also an alternative schedule
where thread B serializes the owner object *before* A starts evaluating
the lazy val. Therefore, forcing B to see the non-evaluating state is
correct.

library/src/scala/runtime/LazyVals.scala

@@ -52,13 +52,29 @@ object LazyVals {
    * Used to indicate the state of a lazy val that is being
    * evaluated and of which other threads await the result.
    */
-  final class Waiting extends CountDownLatch(1) with LazyValControlState
+  final class Waiting extends CountDownLatch(1) with LazyValControlState {
+    /* #20856 If not fully evaluated yet, serialize as if not-evaluat*ing* yet.
+     * This strategy ensures the "serializability" condition of parallel
+     * programs--not to be confused with the data being `java.io.Serializable`.
+     * Indeed, if thread A is evaluating the lazy val while thread B attempts
+     * to serialize its owner object, there is also an alternative schedule
+     * where thread B serializes the owner object *before* A starts evaluating
+     * the lazy val. Therefore, forcing B to see the non-evaluating state is
+     * correct.
+     */
+    private def writeReplace(): Any = null
+  }
 
   /**
    * Used to indicate the state of a lazy val that is currently being
    * evaluated with no other thread awaiting its result.
    */
-  object Evaluating extends LazyValControlState
+  object Evaluating extends LazyValControlState {
+    /* #20856 If not fully evaluated yet, serialize as if not-evaluat*ing* yet.
+     * See longer comment in `Waiting.writeReplace()`.
+     */
+    private def writeReplace(): Any = null
+  }
 
   /**
    * Used to indicate the state of a lazy val that has been evaluated to

tests/run/i20856.check

@@ -0,0 +1 @@
+succeeded: BOMB: test

tests/run/i20856.scala

@@ -0,0 +1,70 @@
+// scalajs: --skip
+
+import java.io.*
+
+class Message(content: String) extends Serializable:
+  //@transient
+  lazy val bomb: String =
+    Thread.sleep(200)
+    "BOMB: " + content
+end Message
+
+object Test:
+  def serialize(obj: Message): Array[Byte] =
+    val byteStream = ByteArrayOutputStream()
+    val objectStream = ObjectOutputStream(byteStream)
+    try
+      objectStream.writeObject(obj)
+      byteStream.toByteArray
+    finally
+      objectStream.close()
+      byteStream.close()
+  end serialize
+
+  def deserialize(bytes: Array[Byte]): Message =
+    val byteStream = ByteArrayInputStream(bytes)
+    val objectStream = ObjectInputStream(byteStream)
+    try
+      objectStream.readObject().asInstanceOf[Message]
+    finally
+      objectStream.close()
+      byteStream.close()
+  end deserialize
+
+  def main(args: Array[String]): Unit =
+    val bytes =
+      val msg = Message("test")
+
+      val touch = Thread(() => {
+        msg.bomb // start evaluation before serialization
+        ()
+      })
+      touch.start()
+
+      Thread.sleep(50) // give some time for the fork to start lazy val rhs eval
+
+      serialize(msg) // serialize in the meantime so that we capture Waiting state
+    end bytes
+
+    val deserializedMsg = deserialize(bytes)
+
+    @volatile var msg = ""
+    @volatile var started = false
+    val read = Thread(() => {
+      started = true
+      msg = deserializedMsg.bomb
+      ()
+    })
+    read.start()
+
+    Thread.sleep(1000)
+    if !started then
+      throw Exception("ouch, the thread has not started yet after 1s")
+
+    if !msg.isEmpty() then
+      println(s"succeeded: $msg")
+    else
+      read.interrupt()
+      throw new AssertionError("failed to read bomb in 1s!")
+  end main
+end Test